A comprehensive study is planned of the properties and mode of action of suppressor T cells from spleen of bursectomized (BX) donors which are capable of transferring agammaglobulinemia to normal histocompatible chickens. The induction of increased suppressor activity in BX chickens by injection of bursa cells and the possible resemblance of this phenomenon to that of allotype and idiotype suppression in rabbits and mice will receive particular attention. Comparison will also be made with the other major type of suppressor cell, induced by regular protein antigens and possibly implicated in the induction of immunological tolerance at the level of T cells responsible for helper effects in antibody production and for delayed hypersensitivity. Thus, a study will be made of tolerance to human gamma-globulin the BX and normal chickens, particularly at the T cell levels, and of the role of antigen-specific suppressor cells in induction and maintenance of such tolerance.